This invention relates to anisotropic microporous membranes comprised of syndiotactic polystyrene and a process for preparing such membranes.
Microporous membranes have long been used in the liquid membrane separation processes of ultrafiltration, microfiltration, membrane distillation, and membrane stripping. Ultrafiltration and microfiltration are pressure driven filtration processes using microporous membranes in which particles or solutes are separated from solutions based on differences in particle size, particle shape, and/or molecular weight. Membrane distillation and membrane stripping are separation processes using microporous membranes in which certain components of the liquid to be treated which are more volatile permeate through the membrane as vapor more rapidly than components which are less volatile due to differences in chemical potential across the membrane. In membrane distillation, the permeated components are condensed on the permeate side of the membrane and removed as liquid, while in membrane stripping the permeated components are removed from the permeate side of the membrane as vapor.
Such microporous membranes may be isotropic or anisotropic (asymmetric). Isotropic microporous membranes possess a morphology in which the pore size within the membrane is substantially uniform throughout the membrane. Anisotropic (asymmetric) microporous membranes possess a morphology in which a pore size gradient exists across the membrane; that is, the membrane morphology varies from highly porous, larger pores at one membrane surface to less porous, smaller spores at the other membrane surface. Such anisotropic membranes thus possess a microporous "skin" of smaller pores. The term asymmetric is often used interchangeably with the term anisotropic.
In the past, such microporous membranes have been fabricated from aliphatic polyolefins such as polyethylene and polypropylene, or from high performance polymers such as sulfonated polyetheretherketone.
However, the aliphatic polyolefin polymers presently used, while inexpensive and easy to process, exhibit relatively low heat distortion temperatures. The high performance polymers, such as sulfonated polyetheretherketone, are derived from polymers which are difficult to process and quite expensive.
What is needed are anisotropic microporous membranes useful for ultrafiltration, microfiltration, membrane distillation, and/or membrane stripping which possess good solvent resistance and heat distortion temperatures, are easily processed, and are prepared from low-cost materials.